Ambulocetus natans: Early to Middle Eocene, above Pakicetus. It
had short front limbs and hind legs adapted for swimming; undulating
its spine up and down helped its swimming. It apparently could walk
on land as well as swim (Thewissen et al. 1994).

Indocetus ramani: earliest Middle Eocene (Gingerich et al. 1993).

Dorudon: the dominant cetacean of the late Eocene. Their tiny
hind limbs were not involved in locomotion.

an early baleen whale with its blowhole far forward and some
structural features found in land animals but not later
whales (Stricherz 1998).

The whale's closest living relative is the hippopotamus. A fossil
group known as anthracotheres links hippos with whales (Boisserie et
al. 2005). The common ancestor of whales and hippos likely was a
primitive artiodactyl (cloven-hoofed mammal); ankle bones from the
primitive whales Artiocetus and Rodhocetus show distinctive
artiodactyl traits (Gingerich et al. 2001).

Gingerich, P. D. et al., 1993. Partial skeletons of Indocetus ramani
[Mammalia, Cetacea] from the Lower Middle Eocene Domanda Shale in the
Sulaiman Range of Punjab [Pakistan]. Contributions from the Museum of
Paleontology of the University of Michigan 28: 393-416.

Gingerich, P. D. et al., 1994. New whale from the Eocene of Pakistan
and the origin of cetacean swimming. Nature 368: 844-847.